In MRI apparatuses, imaging of a subject is attained by irradiating the subject stayed in a uniform static magnetic field generated by a static magnetic field magnet with electromagnetic waves to induce excitation of nuclear spins in the subject, receiving electromagnetic waves generated by the nuclear spins, i.e., magnetic resonance signals, and processing the signals. The irradiation of the electromagnetic waves and reception of the magnetic resonance signals are performed by a device for transmitting and receiving electromagnetic waves of radio frequency (RF), which is called RF antenna or RF coil.
RF coils are roughly classified into two kinds of those called surface antennas or local antennas, and those called volume coils or volume antennas. The local antennas have a round shape or tabular shape, and show sensitivity in a region near the antennas, and they are used by being put on surface of a subject in many cases. On the other hand, the volume antennas have a cylindrical shape or a shape of two discs disposed upper and lower sides, and they show sensitivity in the whole volume in the cylinder or between the discs, and used by placing a subject in that space.
Examples of the volume antennas having a cylindrical shape include those of birdcage type (refer to, for example, Non-patent document 1 and Patent document 1), and those of TEM type (refer to, for example, Patent documents 2 and 3). In these volume antennas, about 16 to 32 conductors of a rod shape usually called rungs (crossbars or rungs of ladder) are disposed in parallel to the center axis of the cylinder and along the side of the cylinder. Such volume antennas in a cylindrical shape are used in an MRI apparatus of the tunnel type. In an MRI apparatus of the tunnel type, a static magnetic field magnet is disposed in a cylindrical shape to form a tunnel, a subject laid on a bed is entered into the inside of the tunnel, and imaging is performed.
Examples of the local antennas include, for example, one consisting of a conductor bent in the shape of a loop (refer to, for example, Patent document 4), one consisting of a conductor bent in the shape of figure eight (refer to, for example, Non-patent document 2), and so forth. Since such local antennas are usually disposed at positions nearer the subject compared with positions at which the volume antennas are disposed in many cases, they provides higher sensitivity compared with the volume antennas, but they can provide only a partial and narrower region for which sensitivity can be obtained in many cases. Therefore, the local antennas are used as a receiving RF antenna in many cases.